Low Impact Development Manual for Michigan - OSEH - University ...

would modestly increase construction costs by
approximately three to five percent As with other
sand filtration systems, steel-enhanced sand filters
should be sized and installed according to local
guidelines, with consideration given to proper
pretreatment for influent solids, as necessary.
7. A gravel layer at least six inches deep is
recommended beneath the filter media.
8. Underdrain piping should be four-inch minimum
(diameter) perforated pipes, with a lateral spacing
of no more than 10 feet. A collector pipe can be
used, (running perpendicular to laterals) with
a slope of one percent. All underground pipes
should have clean-outs accessible from the surface.
Underdrain design must minimize the chance of
clogging by including a pea gravel filter of at least
three inches of gravel under the pipe and six inches
above the pipe.
9. Infiltration filters should be underlain by a layer of
permeable nonwoven geotextile.
10. A total drawdown time of not more than 72 hours
is recommended for constructed filters, though
the surface should drawdown between 24 and
48 hours. The drawdown time can be estimated
using the filter surface area and the saturated
vertical infiltration rate of the filter media. If the
storage does not drawdown in the time allowed,
adjust pretreatment depth, filter media depth, and
surface area. Adjust the design until the volume (if
applicable) and drainage time constraints are met.
11. The filter surface area may be estimated initially
using Darcy’s Law, assuming the soil media is
saturated:
A = V X d /[k x (h + d ) x t ]
f f f f
A = Surface area of filter (square feet)
V = Water volume (cubic feet)
d = Depth of filter media (min 1.5 ft; max 2.5 ft)
f
t = Drawdown time (days), not to exceed 3 days
f
h = Head (average head in feet; typically ½ of the
f
maximum head on the filter media, which is
typically ≤ to 6 ft)
k = Hydraulic conductivity (ft/day)
12. For vegetated filters, a layer of nonwoven
geotextile between non-organic filter media and
planting media is recommended.
13. Filters, especially those that are subsurface, must
be designed with sufficient maintenance access
(clean-outs, room for surface cleaning, entry space,
etc.). Filters that are visible and simple in design
are more likely to be maintained correctly. For
underground vault heights greater than four feet,
ladder access is necessary.
14. In areas where infiltration is infeasible due to a
hot spot or unstable fill that threatens an existing
structure, specify an impervious liner.
Placement of a pipe distribution network in a peat filter
Source: University of Minnesota, NERC
Stormwater Functions and
Calculations
Volume reduction
If a filter is designed to include infiltration, the infiltration
BMP should be followed. There is minimal, if
any, volume reduction for filters that are not designed
to infiltrate.
Peak rate mitigation
Constructed filters generally provide little, if any,
peak rate reduction. However, if the filter is designed
to infiltrate, then medium to high levels of peak rate
attenuation can be expected. Also, as stated above, the
selected media particle size determines the travel time
in the filter and therefore might play a role in meeting
release rate requirements. (See Chapter 9, LID
Stormwater Calculations and Methodology, for more
information on peak rate mitigation).
LID Manual for Michigan – Chapter 7 Page 164